In an optical communication system, a transimpedance amplifier may be used to amplify an electrical current and convert the electrical current into a voltage. A transimpedance amplifier fabricated in silicon CMOS or bipolar technology, for example, may be provided in an optical receiver along with a photodetector.
FIG. 1 shows an exemplary embodiment of an optical receiver in an optical link. As shown in FIG. 1, an optical receiver 100 may comprise of a photodetector 300, and a transimpedance amplifier 400 connected to the photodetector 300 via wirebonds 210.
As shown in FIG. 1, the photodetector 300 receives optical input and converts the light to a proportional photocurrent. The photocurrent is input to the transimpedance amplifier 400 via an electrical connection 210, where it is converted to a peak-to-peak voltage Vout that is conditioned and amplified.
As a current trend, optical receivers are widely used in optical links. Many optical links operate at wavelengths of 1.3 um or 1.55 um using single mode optical fiber. Optical links may be manufactured at lower costs using multimode fiber and using a wavelength of 0.85 um. In these optical links, the requirements of the photodetectors in the optical receiver include, for example:                high speed, of typically 1-10 Gb/s;        high quantum efficiency of greater than 75%;        large area of 50-75 um diameter, since the core diameter of multimode fiber is 50 um or 62.5 um; and        low bias voltage of 2-5V.        
Given the large absorption length of silicon at a wavelength of 0.85 um, and since silicon is not sensitive to the single-mode fiber wavelengths of 1.3 um and 1.55 um, it is extremely difficult for a silicon photodetector to meet all of the requirements for operating in an optical link. For example, since all CMOS and bipolar processes fabricate the active devices very close to the semiconductor surface, such as within 1.0 um, it is difficult to maintain a high quantum efficiency of greater than 75%.
Accordingly, in optical receivers, photodetectors may be fabricated from materials other than silicon. In such cases, the transimpedance amplifier and the photodetector are fabricated separately, and then packaged together with wirebonds or flip-chip attachment after fabrication in the amplifier circuit.
FIG. 2 shows an exemplary embodiment of a transimpedance amplifier for use in optical receivers where the transimpedance amplifier and the photodetector are fabricated separately. As shown in FIG. 2, the transimpedance amplifier 2000 comprises a substrate 200, and power supply 500, and ground 600, and amplifier circuit 400 formed on the substrate 200. Further, as shown in FIG. 2, a photodetector wiring bond pad 100 for wirebonds or flip-chip attachment to the photodetector after fabrication is also fabricated on the transimpedance amplifier 2000.
In these embodiments, at least due to a lack of high speed current sources to drive the input of the transimpedance amplifier, the transimpedance amplifier 2000 is not tested at high speed until it is diced and packaged with a photodetector. In addition, because one and two dimensional arrays of optical receivers in new packaging technologies, such as silicon carrier, may produce individual contact pads as small as 15 um diameter on a pitch of 30 um, and since high speed probes are typically limited to a minimum pitch of 50-100 um, wafer level probing of the high speed output signals may become extremely difficult. Therefore, the transimpedance amplifier 2000 is tested after being packaged with a photodetector. Deferring testing to this stage of fabrication is undesirable since fallout of parts at this stage may be very costly.